Thursday night’s Republican presidential primary debate in Sioux City, the last before votes are cast next month in Iowa, played out as a microcosm of the current race. As his rivals’ attack ads are beginning to chip away at Newt Gingrich’s lead in the key early state, Mitt Romney is making an aggressive case that his general election potential makes him the only real choice for his party’s nomination. And while Gingrich can hold his own on the campaign trail or the debate dais, forces are clearly beginning to align against him.

For two hours, Romney answered almost every question as if he were already the Republican nominee. He routed every answer back to President Obama, unleashing a volley of sharp general election barbs. Question on the economy? “This President doesn’t know how the economy works.” On Iran? Obama espouses “a foreign policy based on ‘pretty please.'” Outlook for the U.S.? “This President thinks America is in decline. It is if he’s President, it’s not if I am.” Even when presented directly with his GOP rivals’ criticisms, Romney deflected. “The President will level the same attacks against me,” he noted, moving on.

And whenever Rick Santorum, Michele Bachmann or Ron Paul spoke, it became clear why Romney could afford to play the “This President” game. From the outset, the three conservative bomb-throwers went after Gingrich, Romney’s greatest primary threat. Santorum wasted no time, raising the attempted conservative “revolt” against Speaker Gingrich in the ’90s with his first answer of the night. Bachmann and Paul then tag-teamed Gingrich on his lucrative consultancy with government-backed mortgage giant Freddie Mac, forcing Newt into uncomfortable territory. “There are a lot of very good institutions that are government sponsored” is a line that could easily come back to bite him, and he tried to escape the whole issue on a technicality, repeating “I never lobbied” over and over. The effect wasn’t flattering. Though Romney’s record on abortion is certainly softer, Bachmann’s pro-life ire was focused on Newt, grasping at his supposed failures to defund Planned Parenthood as Speaker or purge pro-choice Republicans from Congress.

Romney’s only major challenge of the night came not from his opponents, but from moderator Chris Wallace, who made the oft-repeated request that Romney explain the politically expedient policy shifts he executed between running for office in Massachusetts and running nationally. Romney gave one of his better answers to date. “In regards to abortion, I changed my mind,” he said, describing his change of heart after studying embryonic stem cell research legislation that he eventually vetoed in 2005. “Where I was wrong, I’ve tried to correct myself.” His answers on gay rights and gun control weren’t as slick–he pledged fealty to “the gun lobby,” a little off target–but he didn’t get testy as he had in a recent Fox News interview.

Another potential boon for Romney was that Rick Perry had a decent night. Gone were the face-palm inspiring gaffes that characterized his fall performances–he even corrected a moderator at one point!–and joked that he hopes to be the “Tim Tebow of the Iowa caucuses.” Romney can only hope. The more Perry et al. splinter the conservative bloc in Iowa and draw votes from Gingrich, the better chance Romney has of uniting the establishmentarians and landing an early knock-out punch.

As he is in the race, the wildcard of the night was Ron Paul. Paul continues to creep up in Iowa polls, commanding a strong second-place position as the caucuses approach. Beside attacking Gingrich, the centerpiece of Paul’s night was an extended back-and-forth with Santorum and Bachmann on Iran. Paul has always preached non-interventionism, but it hasn’t been featured so prominently in any other debate. Whether that helps his cause in Iowa or hurts it is not clear. There’s definitely a growing isolationist streak in the GOP and it never fails to fire up his youthful fan base, but it’s well outside the GOP mainstream. Iowa voters who find Paul’s fiscal principles appealing may have second thoughts after this kind of exposure to his foreign policy views. But it’s difficult to say.